The user enters the target's subconscious by placing a hand on the target's head and extracts any information needed, even if the target has forgotten it or has had it blocked. A special device can be used to decrease the time it takes to read the target's mind and allows other ninja to assist. Memories that have mental blocks on them will require longer periods of time to probe through and obtain however. Depending on the skill of the user and their level of experience, they will be able to go through days worth of the target's memories in a matter of moments and sift through until they find what they want while other may have to go through each memory in its entirety.
Nagato, through his Human Path, was also able to read minds simply by placing his hand on his target's head. This was effective as it was quick and did not require any devices or machines but had the drawback of permanently extracting the victim's soul afterwards.
In Naruto: Shippūden episode 227, Aoba demonstrated the ability to read a targets mind without actually coming into direct physical contact with them.